Pressure gauges enjoy very extensive commercial and industrial use and are consequently regarded as high production items. Because of such wide use, they are supplied by a plurality of manufacturers and sold in very price conscious competition. Each manufacturer instinctively strives to reduce product costs by improvements, however marginal, which reduce labor and/or materials that can contribute to cost savings in the end product.
Commonly affording pressure sensitivity in the pressure gauge is a Bourdon tube of a pressure-tight construction having a free end displaceably movable in a well known and predictable manner in response to pressure changes supplied at its inlet. To translate tube movement into values of pressure, a pointer is displaceably driven by its free end opposite a calibrated dial plate. For ensuring readout accuracy, provision is usually made for calibrating the pointer to the zero position of the dial.
In most conventional gauge constructions, the inlet end of the Bourdon tube is secured to a stem connection extending outward of the casing either rear or bottom. In the back connected version the dial plate can be rotatably positioned in the course of assembly relative to the socket to effect the zero adjustment without detrimentally affecting the assembly. This is not possible, however, in the lower or bottom connected design since the dial plate aligns with the socket and cannot be adjusted. It has also been known to rotate the coil/pointer/crystal as a unit in effecting the zero calibration by use of a crystal construction as disclosed, for example, in U.S. Pat. No. 3,952,598. The latter has been found particularly suitable and economically advantageous as a manufacturing adjustment for a relatively small diameter fire extinguisher type gauge in which the pointer is directly supported on the free end of the coiled Bourdon tube. Such constructions, however, do not permit post-assembly calibration. Moreover, they generally expose the Bourdon tube and appearance-wise are marketably unsuitable for larger diameter, conventional type gauges in which only the pointer is usually visible through the crystal while the Bourdon tube and movement, if any, are concealed. It has therefore not been known how to construct a zero adjustment on such gauges capable of utilization either during or post-assembly while being equally suited for either a lower or back connected socket construction. Despite recognition of the problem, a ready solution has not heretofore been known.